This present disclosure relates to toners and developers containing the toners for use in forming and developing images, and in particular to toners formed using purified polyester resins. The disclosure also relates to processes for producing and using such toners and developers.
In electrophotographic printing processes, a photoreceptor containing a photoconductive insulating layer on a conductive layer is imaged by uniformly and electrostatically charging the surface of the conductive layer. By exposing the photoreceptor to a pattern of activating electromagnetic radiation, such as light, the radiation selectively dissipates the charge in illuminated areas of the photoconductive insulating layer, while an electrostatic latent image is formed on the non-illuminated areas. Toner particles are attracted from carrier granules to the latent image to develop the latent toner image. The toner image is then transferred from the photoconductive surface to a sheet and fused onto the sheet.
Various toner compositions for such a printing system have been produced using a wide array of additives and constituent materials. Generally, toner particle compositions include a binding material, such as a resin, and any of various additives, such as colorants and waxes, to provide particular properties to the toner particles.
Numerous devices and processes are used to prepare toner particles. Examples of commercially known processes include the melt-blending of toner components in a Banbury roll mill apparatus, spray drying, dispersion polymerization, solution polymerization, and the like. An additional device and process that may be used to prepare toner compositions is a melt extrusion apparatus and process, which possesses a number of advantages over a Banbury roll mill apparatus and process. For example, melt extrusion is a continuous process, rather than a batch process, and extrusion processes can be easily automated, allowing for more economical toner preparation. Examples of conventional toners produced via melt extrusion are described, for example, in U.S. Pat. Nos. 4,894,308, 4,973,439, 5,145,762, 5,227,460, 5,376,494 and 5,468,586, the entire disclosures of which are incorporated herein by reference.
Emulsion aggregation toners are also excellent toners to use in forming print and/or xerographic images in that the toners can be made to have uniform sizes and in that the toners are environmentally friendly. U.S. patents describing emulsion aggregation toners include, for example, U.S. Pat. Nos. 5,370,963, 5,418,108, 5,290,654, 5,278,020, 5,308,734, 5,344,738, 5,403,693, 5,364,729, 5,346,797, 5,348,832, 5,405,728, 5,366,841, 5,496,676, 5,527,658, 5,585,215, 5,650,255, 5,650,256, 5,501,935, 5,723,253, 5,744,520, 5,763,133, 5,766,818, 5,747,215, 5,827,633, 5,853,944, 5,804,349, 5,840,462, and 5,869,215, the entire disclosures of which are incorporated herein by reference.
Emulsion aggregation techniques typically involve the formation of an emulsion latex of the resin particles, which particles have a small size of from, for example, about 5 to about 500 nanometers in diameter, by heating the resin, optionally with solvent if needed, in water or by making a latex in water using an emulsion polymerization. A colorant dispersion, for example of a pigment dispersed in water, optionally also with additional resin, is separately formed. The colorant dispersion is added to the emulsion latex mixture, and an aggregating agent or complexing agent is then added to form aggregated toner particles. The aggregated toner particles are heated to enable coalescence/fusing, thereby achieving aggregated, fused toner particles.
Two main types of emulsion aggregation toners are known. First is an emulsion aggregation process that forms acrylate based, for example, styrene acrylate, toner particles. See, for example, U.S. Pat. No. 6,120,967, the entire disclosure of which is incorporated herein by reference, as one example of such a process. Second is an emulsion aggregation (EA) process that forms polyester, for example, sodio sulfonated polyester, toner particles. See, for example, U.S. Pat. No. 5,916,725, the entire disclosure of which is incorporated herein by reference, as one example of such a process. Alternatively, toner particles can be formed via an EA process that uses preformed polyester latex emulsions made using solvent flash or phase inversion emulsification such as those toner methods described in U.S. Patent Application Publication No. 2008/0236446, the entire disclosure of which is incorporated herein by reference. Additionally, so-called ultra low melt polyester toners can be obtained by incorporation of a suitable crystalline polyester. Examples of EA ultra low melt (ULM) polyester toners, such as those described in U.S. Pat. Nos. 5,057,392, 5,147,747, 6,383,705, 6,780,557, 6,942,951, 7,056,635 and U.S. Patent Application Pub. No. 2008/0236446, the disclosures of which are incorporated by reference in their entirety.
Polyester-based toners (both conventionally extruded and emulsion aggregation based) have recently begun to replace styrene-acrylate toners due to the lower achievable minimum fixing temperatures (MFT) of polyester-based toners. Lower MFT toners provide the opportunity for higher print productivity and/or reduced fusing temperatures, and therefore lower printer power consumption. Polyesters may be prepared via step-growth polycondensation of di-acid and diol. To obtain a high molecular weight polyester from such a polycondensation reaction typically requires high temperature and vacuum removal of the alcoholic by-products. As the molecular weight of the polyester increases, the viscosity also increases dramatically. This viscosity increase can result in imprecise process control, and as a result, the polyester typically has a broad molecular weight distribution. Examples of ultra low melt (ULM) toners, such as those described in U.S. Pat. Nos. 4,246,332, 4,980,448, 5,156,937, 5,202,212, 5,830,979, 5,902,709 and 6,335,139, and U.S. Patent Application Pub. No. 2007/0248903, the disclosures of which are incorporated by reference in their entirety, are prepared by numerous methods.
While toners comprised of these resins may exhibit excellent fusing properties including lower crease MFT and broader fusing latitude, problems such as poor toner flow, relatively low toner blocking temperatures, high triboelectric charging sensitivity with respect to changes in humidity and poor printer fuser life may still exist. The present inventors believe these problems may be due to the presence of a large amount of low molecular weight materials present in the polyester resin. The low molecular weight materials of the polyester resin are typically comprised of di-acid and di-hydroxyl monomers and short chain-length oligomers of these monomers. These low molecular weight materials typically are relatively volatile at the high temperature conditions associated with the fuser and thus may lead to undesirable chemical reactions occurring in-situ in the fusing apparatus. For example, during image fixing at high temperature conditions, the free polyvalent acid monomers (the unpolymerized monomer species) can react with the fuser oil and/or certain additives within the toner to produce contaminants that can deposit on the fuser roll, such as zinc salt contaminants. The buildup of these contaminants signficantly reduces the number of defect-free prints a xerographic device can output before replacement of the fuser roll is required. The inventors further believe problems, such as as poor toner flow and blocking, may be associated with the propensity of the contaminants to plasticize the toner particle and therefore result in a lowering in the Tg (glass transition temperature) of the toner. Further, the presence of low molecular weight acid monomers and oligomers are believed to result in an increased propensity to absorb moisture and therefore affect the variable charging performance as a function of the ambient humidity level.